1. Technical Field
The present invention relates to a semiconductor integrated circuit device. More particularly, the present invention relates to a semiconductor integrated circuit device having as a fuse device a non-volatile memory cell transistor fabricated by a volatile semiconductor memory process.
2. Description
Semiconductor memory devices are generally classified into volatile semiconductor memory devices and non-volatile semiconductor memory devices. The volatile semiconductor memory devices may be subdivided into dynamic random access memory devices and static random access memory devices. A volatile semiconductor memory device has the characteristic of losing contents stored in memory cells when an external power source supply is cut off.
Meanwhile, non-volatile semiconductor memory devices may be divided into mask read only memories (MROMs), programmable read only memories (PROMs), erasable programmable read only memories (EPROMs), electrically erasable programmable read only memories (EEPROMs), etc.
Such non-volatile semiconductor memory devices can permanently preserve the contents stored in the memory cells even though the external power source supply is cut off, thus the devices are usually used to store data to be preserved regardless of the supply of a power source. However, in general, users find it difficult to freely perform erase and write operations or programming operations with an MROM, PROM, or EPROM by itself in an electronic system. That is, it is not easy to erase or re-program the programmed contents in an on-board condition. Meanwhile, in the case of an EEPROM, the erase and write operation can be electrically performed in the system itself, thus its application to a system program storage device or auxiliary memory device necessary for a consecutive contents update is being increased gradually.
FIG. 6 shows a block diagram of a memory device including a volatile memory cell array, such as a DRAM (Dynamic Random Access Memory), or an SRAM (Static Random Access Memory), etc. The memory device also includes a redundancy memory cell array for replacing defective memory cells, memory cell address decoders, and a defective memory cell address storage circuit for storing addresses of defective memory cells.
In the meantime, a semiconductor integrated circuit device having a volatile memory cell array has generally employed a fuse for fault recovery (hereinafter, referred to as a “fuse device”) in the defective memory cell address storage circuit in order to account for an electrical characteristic fault or defect of a memory cell. A polysilicon fuse composed of a polysilicon layer has been initially used as such a fuse device, and has been manufactured together in a process of forming a gate electrode of a metal oxide semiconductor field effect transistor (MOSFET), or a wiring layer.
Generally, in the past the fuse device has been cut by a light source, such as a laser beam, etc. However, such a laser cutting process cannot be performed on a packaged device, and must therefore be performed separately at the wafer or chip level. Further, if polysilicon melted in the laser cutting process remains in the neighborhood of the cut portion, it affects the fuse adjacent thereto, or the cut fuse may operate like a re-connected fuse, which may degrade the reliability of the fuse cutting process.
To solve such problems, a fuse cutting method using a current to “blow” or cut the fuse has been also used in this field, as shown in FIG. 1. In the example of FIG. 1, if the redundancy enable signal PREi is in a logical low (“L”) state, it indicates that a defective address is not stored, and if it is in a logical high (“H”) state, it indicates that an address of a defective memory cell is stored. However, if desired, the opposite logic levels or states could be used instead.
Referring to FIG. 1, the resistance value of a fuse device F1 made of a polysilicon layer is set much lower than a resistance value of the fixed resistance R. Thus, at an initial state when a supply voltage EVCC is applied, an address signal ADDR is applied as a low state, and an external drive signal MRS is applied as a high state, then the logic level of a node A becomes high (“H”) and the logic level of a node B becomes low (“L”). Accordingly, the logic level of a redundancy enable signal PREi outputted from an output inverter IN2 becomes “L.”
Meanwhile, in case a defective memory cell should be repaired, the supply voltage EVCC is applied and the address signal ADDR is provided as a high state. Thus, an N-type MOS transistor N1 for blowing the fuse F1 is turned on, and then an over-current starts flowing through the fuse device F1. That is, the fuse device F1 is broken by the over-current flowing from the fuse device F1 to a source terminal of the N-type MOS transistor N1. If the fuse device F1 is blown by the current, then the supply voltage EVCC cannot be applied to the source of the P-type MOS transistor P1 regardless of the state of the address signal ADDR, but is applied only through the resistance R. Therefore, when the drive signal MRS is applied as a high state, a logic level of the node A becomes “L” and a logic level of the node B becomes “H”. Thus, a logic level of the redundancy enable signal PREi is outputted as “H”, to repair a defective memory cell with a redundancy memory cell.
However, though the current blowing method referred to with respect to FIG. 1 has an advantage that the method can be used for a device in either a packaged state or a wafer state, there are still problems wherein the fuse device is re-connected after a breakage of the fuse device, or that layers near the fuse device are damaged by the breakage of the fuse.
To try to solve the breakage problem of the fuse device, an N-channel MOSFET having a floating gate has been provided as a fuse device in a volatile semiconductor memory device. In this case, a cutting operation of the transistor as the fuse device is performed by irradiating an electron beam generated by an electron beam apparatus through an opening part of an isolation layer formed on an upper part of the floating gate. That is, when electrons are injected into the floating gate, a threshold voltage of the N-channel MOSFET is varied. Thus the transistor functions as a fuse having an electrically cut state.
However, under the programmed state wherein the electrons are injected into the floating gate through the irradiation of the electron beam, it is difficult to return to the original state. The N-channel MOSFET has no control gate, like a memory cell transistor of an EEPROM has, and thus program and erase operations can not be performed with an applied voltage. Therefore, the N-channel MOSFET having the floating gate cannot perfectly perform an operation, like the memory cell transistor of an EEPROM.
A technique is therefore required that is-capable of easily fabricating a non-volatile memory cell as a fuse device that can perform an operation like the memory cell transistor of an EEPROM, using a fabrication process of a volatile semiconductor memory such as a DRAM, an SRAM, etc.
However, a general non-volatile semiconductor memory, e.g., a memory cell transistor of an EEPROM, requires a high voltage, e.g., about 10-18 volts for a programming voltage, and needs a read voltage of about 5 volts. Thus, a chip internally employs a high voltage pump circuit, and a memory cell transistor has a floating gate surrounded with a dielectric film and has a control gate formed on an upper part of the floating gate, which is quite different from a general MOSFET. That is, such a process of fabricating the non-volatile semiconductor memory is much different from a fabrication process of a volatile semiconductor memory cell. There are many difficulties to manufacture the memory cell transistor of an EEPROM under the fabrication environment of the volatile semiconductor memory device such as a DRAM, etc., without an additional process or a change of the process.
Accordingly, it would be desirable to provide a fuse device of a volatile semiconductor memory device capable of solving problems of the prior art.
It would further be desirable to provide a semiconductor integrated circuit device having as a fuse device a non-volatile memory cell transistor fabricated in a fabrication process of a volatile semiconductor memory.
It would further be desirable to provide a fuse device that can be electrically cut without a breakage caused by using a laser beam or current.
It would still further be desirable to provide a method of manufacturing a fuse device through a fabrication process of a volatile semiconductor memory, and a structure of the fuse device therefor, which is capable of operating like a memory cell transistor of an EEPROM.
It would additionally be desirable to provide a structure of a fuse device capable of performing an operation like a memory cell transistor of an EEPROM at a voltage lower than an operating voltage of typical, existing EEPROMS.
It would still additionally be desirable to provide a structure of an EEPROM memory cell transistor as a fuse device capable of performing an operation like the memory cell transistor of the EEPROM within an operating voltage range of a volatile semiconductor memory device.
Furthermore, it would also be desirable to provide a non-volatile memory cell transistor having a single-polysilicon-layer structure in which a fuse device can be cut without a breakage, being fabricated under a DRAM fabrication environment, and that can be repaired to an original uncut state when necessary.
Moreover, it would be desirable to provide a structure of an EEPROM memory cell transistor used as a fuse device, whereby an address of a defective bit is stored, or a mode entry signal indicating an entry into a specific operating mode such as a test mode etc., is stored.
It would be yet further desirable to provide a semiconductor integrated circuit device having a fuse device that can be cut without a breakage and repaired even while in a packaged state.
It would be yet still further desirable to provide a defective memory cell address storage circuit of a semiconductor integrated circuit device having as a fuse device a non-volatile memory cell transistor manufactured in a fabrication process of a volatile semiconductor memory.
To these ends, according to one aspect of the present invention, an integrated circuit device having a semiconductor substrate comprises: a memory cell array comprising a plurality of volatile memory cells; and a metal oxide semiconductor field effect transistor (MOSFET) configured as a non-volatile electrically erasable programmable read only memory (EEPROM) device having a floating gate and a control gate, and adapted to store status information associated with the memory cell array, wherein the MOSFET has a single polysilicon film for the floating gate, and the control gate is disposed within a well in the semiconductor substrate.
The single polysilicon layer of the MOSFET corresponds to a charge storage floating gate of an EEPROM, and a control gate of the EEPROM is spaced from a channel region of the MOSFET and can be composed of an ion-implantation region formed beneath the single polysilicon layer. The MOSFET can be an n-channel MOSFET, in which case a programming operation for electrically blowing or cutting the fuse device can be performed by injecting electrons into the floating gate through a hot electron injection method. Further, an erase operation for electrically connecting the fuse device can be performed by discharging electrons captured by the floating gate through an F-N (Fowler-Nordheim) tunneling system.
According to another aspect of the present invention, an integrated circuit device having a semiconductor substrate comprises: a memory cell array comprising a plurality of volatile memory cells; at least one redundancy volatile memory cell; and an electrically programmable non-volatile memory cell configured as a fuse device to identify whether a corresponding one of the memory cells is defective and should be replaced with the redundancy memory cell.
According to yet another aspect of the present invention, an integrated circuit device, comprises: a plurality of volatile memory cells; a redundancy memory-cell; and a defect repair circuit for replacing a defective memory cell, among the plurality of volatile memory cells, with the redundancy memory cell, wherein the defect repair circuit includes a fuse device comprising a MOSFET having a floating gate and a control gate, said MOSFET being changed from a first threshold voltage level to a second threshold voltage level in response to an applied programming control signal, and being changed from the second threshold voltage level to the first threshold voltage level in response to an applied erase control signal, to electrically disconnect and connect the fuse device, and wherein the MOSFET has a single polysilicon film for the floating gate, and the control gate is disposed within a well in the semiconductor substrate
According to still another aspect of the present invention, a defective memory cell address storage circuit for a semiconductor integrated circuit device comprises: a MOSFET, having a source, a drain connected to a first power supply voltage, a floating gate, and a control gate connected to a second power supply voltage, the MOSFET being configured as a fuse device for storing status information for the integrated circuit device; an operation enabler for connecting a source of the MOSFET to a ground voltage in response to a status of an enable signal; and a latch for latching, as the status information, a voltage level appearing at the source of the MOSFET according to a threshold voltage of the MOSFET, to store a defective memory cell address, wherein the MOSFET has a single polysilicon film for the floating gate, and the control gate is disposed within a well in the semiconductor substrate.
The MOSFET, configured as a single-polysilicon-layer EEPROM-type cell, functions as a fuse device for storing status information. A drain is connected to a first power supply voltage and a control gate is connected to a second power supply voltage. Further, the MOSFET is manufactured through a fabrication process of a volatile semiconductor memory.
The operation enabler connects a source of the MOSFET to a ground voltage in response to a state of an enable signal.
The latch latches, as the status information, a voltage level appearing at a source of the MOSFET according to the threshold voltage of the MOSFET, to indicate whether an address of a defective memory cell is stored.
The MOSFET of the single-polysilicon-layer EEPROM-type cell is manufactured by the fabrication process of a volatile semiconductor memory.
According to a further aspect of the present invention, a dynamic random access memory device comprises: a MOSFET configured as a fuse device for storing status information for the memory device, said MOSFET including, a substrate having a first conductivity type; a first well formed in a first portion of the substrate and having a second conductivity type; a pocket well formed within the first well, the pocket well having the first conductivity type; source and drain regions formed in a portion of the pocket well; a second well formed in a second portion of the substrate, being spaced from the first well, the second well having the second conductivity type; a control gate formed in a portion of the second well; a floating gate of polysilicon material at least partially extending over and overlapping the control gate, extending above and overlapping portions of the pocket well and the first well in a direction approximately perpendicular to a source-drain channel between the source and the drain; and a tunnel oxide interposed between the floating gate and the substrate.
Further, the erase operation is obtained, through a grounding of the source of the MOSFET, and by applying the first and second power supply voltages of about 5 volts and 0 volts, respectively. The read operation is performed by floating of the source of the MOSFET and by applying both the first and second power supply voltages of about 2.0-3.0 volts.
The operation enabler may be provided as an N-type MOS transistor, and the latch can be an inverter latch whose input terminals are connected together with its output terminals. An output inverter for inverting and outputting an output of the latch can be further equipped therewith.
Herewith, the second conductivity type can be an n−type conductivity while the first conductive type is p-type conductivity.